Extensive use of electron beam metrological systems is made to measure the lengths of circuit patterns formed on wafers and inspect them. One example of such electron beam metrological systems is disclosed in U.S. Pat. No. 4,767,926. This known system starts the measurement of the length of a specimen such as a wafer by digitally scanning the specimen with an electron beam. Secondary electrons are produced from the specimen and detected by a detector. The output signal from the detector is sent to a memory, where the signal is stored. Data is read from the memory to determine the length. As an example, when the width of a pattern of lines is to be measured, the signal read from the memory is processed to determine the addresses corresponding to the left edge and the right edge, respectively, of the pattern. In particular, the addresses at which the locally maximum values of the signal are stored are found to determine the addresses corresponding to both edges. Then, the difference between these two addresses is calculated. The length is determined, using this difference and the magnification of the image or other factor.
As semiconductor chips are manufactured at higher density, electron beam metrological systems have been required to measure lengths at higher resolution. One of the factors which determine the resolution of the metrology is the number of steps in the stepwise scan made in the X direction. Of course, the diameter of the electron beam also affects the resolution, but where the diameter is sufficiently small, if the number of the steps of the scan made in the X direction is doubled, then the resolution is improved by a factor of 2.
However, increasing the number of steps of the scan presents problems. As an example, if the number is increased from 512 to 1024, then an expensive monitor display with 1024.times.1024 pixels is needed. Also, an expensive frame memory and an expensive scanning system for the electron beam directed to the specimen are necessitated. Therefore, the conventional electron beam metrological system for high-resolution metrology is expensive to fabricate.